Fixed/absolute layouts
By default layouts are fixed using absolute positions. This means that you can place objects anywhere you like, so you have complete control over the layout. The downside of this layout mode is that it will be harder to make the page responsive because the layout will only look perfect on the screen size it was designed for.
And since there is no 'structure' in the layout (objects can be placed anywhere in random order), there is no way for the browser to automatically re-position and/or scale the content based on the view port.
So to make sure the page content looks good on different viewports (screen sizes), you will have to implement breakpoints (different layout variations) of the same page.

Flexible/floating layout
You can implement flexible layouts by using Layout Grids. This make it possible to create Bootstrap-like layouts. When using layout grids, the layout is controlled by the grids. Objects will automatically resize based on the available space in the viewport and the grid column size.
The downside of using layout grids is that you have less control over the layout. All objects will be positioned and resized automatically. So you are forced into a structured layouts.